1. Field of the Invention
The present invention relates to a filter device and, more particularly, to a ladder filter for cleaning dampening water used in offset printing.
2. Description of the Related Art
In offset printing, the offset press is supplied with a liquid which contains various chemicals. This liquid is generally referred to as "dampening water". During the operation of the offset press, the dampening water is inevitably contaminated by printing ink and paper dusts. Since the contaminated dampening water deteriorates the printing surface of the press undesirably, it is essential to clean the dampening water by a suitable means.
An apparatus called "ladder filter" is known as an apparatus for cleaning the dampening water. As shown in FIG. 6, a typical conventional ladder filter has a tank 30 in which are formed a stirring section 32, a floating section 34 and a discharge section 36 through which the contaminated dampening water flows successively such as to be cleaned before it reaches the discharge section 36.
The dampening water is bubbled by a bubbling plate 40 before it flows into the stirring section 32 of the tank 30. The ink, paper dusts and other contaminants (referred to as "contaminants" inclusively, hereinunder) suspended by the dampening water attach to the bubbles in the stirring section 32. The bubbles tend to gather around a first partition plate 42 and a second partition plate 44 which define respective sections in the tank 30, and the water is cleaned by removing these bubbles. In the conventional ladder filter, however, the removal of the bubbles is done by human labour at a suitable period, so that the cleaning efficiency is impaired. In addition, there is a tendency that the painful work for removing the bubbles is often ignored resulting in an imperfect cleaning of the dampening water.
On the other hand, Japanese Utility Model Laid-Open No. 39649/1984 discloses a ladder filter having a floating matter removing pipe which opens at a level slightly below the liquid level in the floating section 34 of the tank 30. In this ladder filter, since the opening through which the floating matters are discharged is positioned below the liquid level, a large quantity of dampening water is discharged together with the floating matters, so that the consumption of the dampening water is increased correspondingly. In addition, the efficiency of removal of the floating matters is rather low because the area of the opening of the removing pipe is small.